Distortion-product emissions (DPEs) have been shown to be sensitive indicators of cochlear function. Several features of these particular otacoustic emissions make them important candidates for the measurement of progressive hearing loss. Thus, DPEs appear to uniquely test outer hair-cell function, they are simply measured by inserting a miniature microphone system into the external ear canal, and they can precisely measure the ear's response to subtle gradations in stimulus frequency and intensity. All these positive features suggest that DPEs may someday become a powerful test of hearing impairment. The outcome of a small pilot study we recently completed on both normally hearing and hearing-impaired ears supports the potential utility of DPEs in a clinical setting. The present study proposes first to extend our initial data base to include a larger sampling of normal hearing subjects so that reliable descriptive statistical analyses can be performed. Sources of individual variability in "normal" ears will be investigated by relating DPEs to other emission types and fine- resolution measures of hearing. In addition, statistical and technical procedures will be investigated to assess their effectiveness in reducing variability in DPE measures and related noise floors in order to increase the dynamic range of the response measure. Secondly, patient populations representing several common progressive-hearing disorders will be tested for DPEs in order to relate these measures to the frequency pattern and degree of hearing loss. An attempt will also be made to determine if the etiology of the hearing disorder significantly influences the DPE results. Finally, selected patients will be monitored over the requested 5-yr project period to establish the ability of DPEs to describe the course of their progressive-hearing losses.